1. Field Of The Invention
The present invention relates to a novel optical element substrate. More particularly, the present invention is concerned with an optical element substrate comprising a random copolymer comprising a methyl methacrylate unit, an aromatic vinyl compound unit, an unsaturated aliphatic acid unit and a hexagonal acid anhydride unit in specific weight proportions, which substrate has excellent heat resistance, thermal stability, transparency, resistance to scratching and metal layer corrosion preventive properties and exhibits advantageously low double refraction and warpage. The term "optical element substrate" as used herein defines a substrate body for an optical element. The term "optical element" as used herein defines all types of elements which utilize light transmission, refraction, reflection and other optical characteristics. Representative examples of optical elements include an optical disc such as a digital audio disc, a video disc and a disc which is capable of being directly read after recording, a mirror, a lens and the like. The optical disc substrate according to the present invention is especially suited for a high-density information medium for use in a recording and playback apparatus. This apparatus converts analogue information into digital information and records the digital information in a recording medium at a high packing density by means of a laser beam. Further, due to the convenience and advantage of the apparatus, it has become the object of public attention.
2. Discussion Of Related Art
The term "optical disc" as mentioned above and used hereinafter defines an information disc which is read optically in reflection, and includes, for example a digital audio disc (DAD), a video disc (VD), a so-called direct read after write (DRAW) disc and a so-called erasable direct read after write (E-DRAW) disc.
In a DAD, audio information, which has been finely divided into signals and converted into a binary number system of "0" and "1", is recorded, on a layer capable of reflecting a laser beam, e.g., a metal layer, as a relief structure having a crenellated profile of areas situated alternately at a higher and a lower level (information bits), sometimes termed blocks and pits. The relief structure is read by means of a laser beam, and the signals of the laser beam, according to the binary number system, are converted into electrical signals and then played back as sound. In general, such a disc having a relief structure of a metal layer capable of reflecting a laser beam, is obtained by molding a transparent resin into a disc-form substrate having a relief structure and, forming on the relief structure surface, a metal layer by vacuum evaporation coating or the like.
Image information can likewise be recorded as a relief structure on a metal layer capable of reflecting a laser beam to give a VD.
Computer programs and data can also be recorded on an optical disc as relief structures, to give an optical disc which is usable as an information disc of computer programs and data. Discs such as information discs for computers or a disc which is capable of being directly read after writing or recording (DRAW disc), are especially drawing attention. Further, an optical disc which is capable of being written on and erased [erasable direct read after write disc (E-DRAW disc)] is now being developed. In the field of DRAW discs, there are employed various types of recording layers. Examples of such recording layers include (1) a layer in which holes are formed by irradiation of a laser beam, (2) a layer in which formation of bubbles by irradiation of a laser beam is utilized, and (3) a layer in which the magnetic direction is changed by irradiation of a laser beam. The recording layer of the above type (3) is usable for E-DRAW discs.
To read the relief structure in the above-mentioned various optical discs by means of a laser beam, the laser beam interference, which is caused by the phase difference between a laser beam directly incident on a detector and a laser beam traversing the substrate and reaching the detector, must be detected. Therefore, the resin to be used as the material for a substrate for optical discs must satisfy the following requirements:
(1) the resin must have a high laser beam transmission;
(2) after the resin has been molded into a disc substrate, the orientation of the molecules in the resin must be small so that the double refraction of the laser beam is small;
(3) after the resin has been molded into a disc substrate, the refraction index of the resin must not vary over the disc;
(4) the resin must not contain contaminants;
(5) the resin must have good heat distortion resistance;
(6) the resin must be susceptible to good vacuum evaporation of a metal;
(7) the resin must have good molding properties and be capable of being molded into a disc substrate with a sharp pit pattern;
(8) the resin must have good adhesion to a reflective layer,
(9) after the resin has been molded into a disc substrate, the thickness must be uniform over the disc; and
(10) after the resin has been molded into a disc substrate, the disc must be stable with respect to freedom from warpage with the passing of time.
In addition to the above requirements, the resin should not contain impurities such as the polymerization solvent, and should not be hydrolyzable.
Heretofore, polymethyl methacrylate (PMMA) has been used as a material for substrates for optical discs. However, the moisture absorption of PMMA is high and, hence, an optical disc utilizing PMMA as the material for its substrate would in time become warped, due to the absorption of moisture by the substrate. In order to eliminate this drawback, there has been proposed an optical disc in which a substrate made from a PMMA resin is coated with a resin having good barrier properties to water, such as polyvinylidene chloride. However, for preparing such a disc, a troublesome step is required.
A polycarbonate resin also has been used as a material for a substrate for optical discs. However, the substrate made of this resin is defective in that it inherently contains impurities such as chromium and dichloromethane which would deteriorate the reflective layer. The contamination of the substrate with chromium is caused during the molding of the resin which is usually conducted at high temperatures. The contamination of the substrate by dichloromethane is caused by the dichloromethane having been used as the polymerization solvent and which has not been removed. Further, the substrate made of the polycarbonate resin, would in time become hydrolyzed, and this would lead to a deterioration of the adhesion of the reflective layer which is adhered to the substrate.
Further, optical discs have been proposed in which a copolymer of methyl methacrylate and styrene (methyl methacrylate/styrene=60/40 or 30/70 by weight) is used as the material for substrates (see Japanese Patent Application Laid-Open Specifications Nos. 57-33446/1982 and 57-162135/1982). However, these substrates are deficient in heat distortion resistance and the double refraction thereof is disadvantageously high.
The present inventors previously proposed an optical disc substrate made of a resin comprising a copolymer of a methacrylic acid ester and a monoalkenyl aromatic compound (see Japanese Patent Application Laid-Open Specification No. 58-88843/1983). This substrate substantially satisfies the standard requirements for DADs having a diameter of 120 mm and a thickness of 1.2 mm. However, it is difficult for this substrate to satisfy the standard requirements for DRAW discs having a diameter of 300 mm and a thickness of 1.2 mm. Illustratively stated, due to the increased diameter of DRAW discs, the distance in which a molten resin, when subjected to injection molding, flows from the center portion to the edge portion of the mold, becomes large as compared with the case of the molding of DADs. Thus, the orientation of the molecules of the molten resin is likely to occur, which leads to an increase in double refraction. Therefore, it is difficult for the substrate made from this resin to have a double refraction as small as 40 nm or less, such being required as the standard value for DRAW discs, and further as small as 20 nm or less, such being required as the standard value for E-DRAW discs.
The present inventors further proposed an optical disc substrate made of a resin comprising a copolymer which is obtained by copolymerizing 40 to 70 parts by weight of methyl methacrylate, 5 to 20 parts by weight of an ester of a methacrylic acid and a saturated aliphatic monohydric alcohol having 3 to 6 carbon atoms, and 25 to 40 parts by weight of a monoalkenyl aromatic compound, the copolymer having a solution viscosity of 3 to 10 cps as measured at 25.degree. C. with respect to a 10% solution of the copolymer in methyl ethyl ketone and having a double refraction of 100 nm or less (see Japanese Patent Application Laid-Open Specification No. 59-108012/1984). This substrate is satisfactory in double refraction and moisture absorption. However, this substrate is deficient in the heat resistance required when a reflective layer is being formed on the substrate by sputtering or the like, and is defective in that creeping is likely to occur when a disc using the substrate is leaned and stored for a long period of time under warm conditions.
Also, as disclosed in Japanese Patent Application Laid-open Specification No. 60-133004/1985, it has been proposed to employ as an optical high-density information recording medium a resin of a copolymer comprising a methyl- methacrylate unit, an ester unit as obtained by esterification of methacrylic acid with a monovalent saturated aliphatic alcohol having 3 to 6 carbon atoms, a monoalkenyl aromatic unit, an unsaturated aliphatic acid unit and/or a hexagonal acid anhydride unit. The deformation temperature of this resin is as low as 70.degree. to 85.degree. C., presumably due to the incorporation into the resin of C.sub.3 -C.sub.6 ester units in an amount as large as from 4 to 16% by weight, based on the weight of the resin. Accordingly, when an optical disc comprising this resin as a substrate is mounted on a hardware having recording and playback functions and operated over a prolonged period of time, thereby causing the temperature of the disc to increase up to, for example, about 60.degree. C. or more, warpage of the disc occurs. This warpage adversely affects the recording and playback of the hardware.
With respect to a mirror, the conventional process for manufacturing the same comprises applying a reflective metal layer to the whole reverse side of a glass substrate according to any one of the vacuum evaporation coating technique, sputtering technique, ion plating technique and the like, optionally followed by further application of a protective layer. Recently, the employment of plastics as a mirror substrate in place of glass has become desired in the art, especially in the field of a mirror for vehicles, due to the advantages of the plastic substrate in lightweightness, mass production facility and impact resistance, as compared with the glass substrate. The demand for a plastic mirror is rapidly increasing since in the case of a plastic mirror, it is more facile to meet the recent requirements for a color mirror and a mirror endowed with anti-glare properties.
A representative example of the plastics employed as a substrate for a plastic mirror is a polymethyl methacrylate resin. However, this resin is by nature hygroscopic. Therefore, the mirror as fabricated by applying a non-hygroscopic reflective layer of a metal to the surface of one side of the polymethyl methacrylate substrate inherently has a problem of warping due to the absorption of moisture from the open side of the mirror remote from the reflective layer. Accordingly, the use of the plastic substrate is limited, and in particular, this substrate has not yet been realized as being acceptable as a mirror substrate in vehicles.
With respect to a lens, as in the case of a mirror, the demand for a plastic product is rapidly increasing due to its advantages in lightweightness and mass productivity. Representative examples of the plastics employed for a plastic lens are a polymethyl methacrylate resin, a polycarbonate resin and an allyldiglycol carbonate resin. The polymethyl methacrylate resin is highly hygroscopic and hence has poor dimensional stability, so that lenses comprising the resin as a substrate cannot be suitably employed in a precision optical instrument. The temperature up to which the polymethyl methacrylate resin is heat resistant is only about 80.degree. to 90.degree. C., even under absolute dry conditions. The temperature is decreased by the absorption of moisture by the resin. Accordingly, when the lens comprised of the resin is used in an environment having an elevated temperature, for example near the lamp of a car, the lens must be thick so as to prevent the lens from being deformed by the heat. The necessity of being thick is disadvantageous from the viewpoint of manufacturing cost. Moreover, the refractive index of the polymethyl methacrylate resin is as low as 1.49. Accordingly, the focal distance of the lens comprised of the resin is of great length, thereby causing the manufacturing of a thin lens to be difficult.
On the other hand, with respect to a polycarbonate resin, it has a drawback in that its light transmission is as low as about 88%. In the polycarbonate resin, a molecular orientation tends to occur, and hence a double refraction disadvantageously tends to occur. The polycarbonate resin has a low hardness in terms of Rockwell hardness or pencil hardness as defined later. Therefore, the lens comprised of the resin is susceptible to scratches, and when it is used for a prolonged period of time, its surface whitens, thereby bringing about the disadvantage of diffused reflection.
An allyldiglycol carbonate resin which is a thermosetting resin is generally used for producing a lens. Due to the thermosetting properties, this resin is accompanied by shrinkage at the time when the lens is being fabricated. Therefore, post-treatment is necessary for the resin. For the foregoing reason, the major use of the allyldiglycol carbonate resin is limited to a lens for glasses. However, this resin is not useful for the fabrication of high-precision, mass-production lenses such as a finder lens, a condenser lens, a floodlight lens, an electric flash diffuser lens and a photographing lens for a camera, and such as a non-spherical surface lens and a television video projector lens which are employed with a compact disc player. The refractive index of the allyldiglycol carbonate resin is as low as about 1.50. Accordingly, as in the case of a polymethyl methacrylate resin, the focal distance of the lens comprised of this resin is of great length, and hence it is difficult to obtain a thin lens by the use of this resin.